A rubber-based hot melt adhesive and a hot-melt adhesive made of a thermoplastic resin have been utilized in applications requiring sealing properties, in various fields such as motor vehicles and electronic materials. Further, a moisture-curable hot melt adhesive has been utilized for adhesion of a seat, an outer skin and the like as an adhesive having high and permanent adhesive properties and flexibility.
Although the moisture-curable hot melt adhesive is used also for a sealing application as an adhesive having high adhesive properties and flexibility, very long curing time has been required because when an uncured adhesive is heated, generation of bubbles due to decarboxylation by a reaction of an isocyanate and moisture as well as expansion will occur. Long curing time is not desirable in terms of inspection and product assurance and has been a hindrance to inspection and shipment.
An inner case and an outer case, a lens and a housing of a lighting appliance and the like are bonded with an adhesive and then subjected to an airtight test. The airtight test is important to verify proper sealing, and is performed immediately after adhesion in many cases. Therefore, a hot melt adhesive for sealing application is required to develop strength by solidification after coating. There has been a problem that a conventional moisture-curable urethane-based hot melt adhesive does not have sufficient early strength by solidification, and bubbles are generated from decarboxylation by a reaction of an isocyanate and moisture as described above when an uncured adhesive is heated.
Patent Literature 1 discloses a reactive hot melt adhesive prepared by blending a blocked isocyanate compound with a modified block polymer in which carboxyl group or acid anhydride group is introduced into a block polymer composed of a polystyrene block (A) and a poly-conjugated diene block or a block of a hydrogenated product thereof (B). However, since the reactive hot melt adhesive in Patent Literature 1 is synthesized by a reaction between an isocyanate and carboxyl group after dissociation of the blocking agent, generation of carbon dioxide is anticipated, and reduction of bubbles required for an adhesive to be used for sealing application cannot be achieved.
Patent Literature 2 describes a moisture-curable hot melt composition containing an isocyanate-terminated polyurethane prepolymer which is a reaction product of crystalline polyester polyol and polyisocyanate, and reactive wax. However, the moisture-curable hot melt adhesive described in Patent Literature 2 cannot suppress sagging, and generation of bubbles and volume expansion by a reaction of an isocyanate and moisture when an uncured adhesive is heated to high temperatures.
In recent years, there has been desired a urethane-based moisture-curable hot melt adhesive for lighting appliances which has rapid-curability and solves the problem of bubble-generation caused by heating the uncured adhesive, and the development thereof has become an urgent requirement.